another consideration is that aluminum heads due to their dissipation of heat can handle a higher compression ratio with crap gas than iron heads, so you can run the same gas with a higher compression ratio. If you have the money, i'd recommend you get the aluminum heads especially if you're going to modify the bottom end or add forced induction. One problem though, due to dissimilar metals is that the head gaskets are more likely to fail if you don't regularly change the coolant (due to the engine grounding the spark, the coolant reacts to the electrical current and becomes acidic which eats a path through the gasket sealing surface. With dissimilar metals the heating and expansion of the metals can stress different parts of the gasket or even cause an opening. If you don't change your coolant often enough, you can hook up a multimeter (Volts) and see the coolant holding a charge - i've seen it, it's damn weird!)
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